Electrostatic actuators have become selected and are the solution of choice for actuators that employ low power, operate at high speed, require low cost to produce, and are of small size. These devices present significant advantages: over thermal devices by requiring much less power; over electromagnetic devices by using less power and having smaller size; over piezoelectric actuators that have a higher cost and have a much smaller amplitude of motion.
To date, however, there are no commercially available electrostatic actuators. Of particular concern are electrostatic actuation in the presence of dielectrically isolated electrodes, where specific problems are incurred.
In electrostatic actuators, the desired displacement is the result of the attractive electrostatic force generated by the interaction between a distribution of opposite sign charges placed on two bodies, at least one of which is moveable. For the purposes of this invention, these two bodies are known as actuator plates. The actuator plates are placed apart by a predetermined distance. The charge distribution is then generated by applying a potential difference between two conductive electrodes that are part of the actuator plates. The actuator will be in the ON state or mode when a potential difference is applied between the electrodes and the two plates move toward each other. The actuator will be in the OFF state when the electrodes are at the same potential (shorted).
A DC voltage is theoretically capable of inducing the "ON" state of the actuation cycle. Practical problems, however, limit the application of a DC voltage for the actuation of some electrostatic actuators. In real devices, DC driving shows memory effects, such that the behavior of the actuators depends strongly on the history of the actuation process. DC driving also induces stiction (1) through charges injected in the dielectric at the dielectric/metal electrode interface, and (2) through charges trapped at the dielectric/air interface.
In our commonly owned copending application entitled DRIVING DEVICE AND METHOD FOR ELECTROSTATIC ACTUATORS, and having Ser. No. 08/948,337, a method and apparatus was proposed for use with an AC signal. The AC signal method of driving an electrostatic actuator was designed for actuators having a pair of actuator plates having electrodes for conducting a voltage potential thereto. At least one of the pair of actuator plates was movable with respect to the other, so that the actuator plates were positioned to move upon application of an AC voltage potential thereto by electrodes attached thereto. The driving device in this application produced a compounded AC signal with a rise section at least 10 to 100 times mechanical response time of the actuator and a fall time at least 10 to 100 times the mechanical response time of the actuator. Advantages of this invention are spelled out in the above referenced copending application. However, the invention worked best in driving arrays of actuators working in parallel.
Descriptions of various prior art patents are also contained therein and are incorporated herein by reference.
There are, however, possible array configurations where the individual cells have one common electrode but have to operate out of phase. If the common electrode is grounded, in order to generate alternate fields between the electrodes of the electrostatic actuator, a dual power supply is needed, providing positive and negative potentials. The relatively high voltages needed for use in electrostatic activators would expectedly involve the use of DC/DC converters to scale up the voltage supplied by a battery or another available voltage. These DC/DC converters are bulky and expensive.
Because of this, use of a single power supply would be of great advantage in designing electrostatic actuators, in part because favorable system size, weight and cost parameters would result. It would be of great advantage to the art to resolve the conflict for operating requirements for electrostatic actuators having different cells using a common electrode with an electrically floating potential.
It would be another great advance in the art if an improved driving method for electrostatic actuators could be provided with a desired alternate polarity fields to all the actuator cells based on a single DC power supply and a single DC/DC converter.
Other advantages will appear hereinafter.